Genesis of the Poroshiri ophiolite, Hokkaido, Japan; Inference from geochemical evidence

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  • Miyashita Sumio
    Department of Geology, Faculty of Science, Niigata University
  • Adachi Yoshiko
    Department of Geology, Faculty of Science, Niigata University Present address: Center for Transdisciplinary Research, Niigata University
  • Tanaka Shinji
    Department of Geology, Faculty of Science, Niigata University
  • Nakagawa Mitsuhiro
    Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University
  • Kimura Jun-ichi
    Department of Geoscience, Interdisciplinary Faculty of Science and Engineering, Shimane University

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Other Title
  • ポロシリオフィオライトの生成場：微量成分組成からの検討
  • ポロシリ オフィオライト ノ セイセイバ ビリョウ セイブン ソセイ カラ ノ ケントウ

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Abstract

Seven samples (four metabasaltic rocks and three meta sheeted-dike samples) and two samples of later metadolerite dikes of the Poroshiri ophiolite were analysed by ICP-MS. Chondrite normalized REE values of the metabasaltic rocks exhibit N-MORB pattern. Multi-element spider diagram shows a N-MORB signature except for variable concentrations in LIL elements that are ascribed to secondary modification. These metabasaltic rocks plot in N-MORB fields in discrimination diagrams using immobile elements and are distinct from such as IAT, BABB and E-MORB. Later dolerite dikes, which intruded after considerable cooling of the ophiolite, have also N-MORB signatures. However, the later dikes are slightly depleted in HREE and enriched in LREE concentrations, suggesting more fertile source than the Poroshiri ophiolite. These geochemical signatures and wide compositional range of the metabasaltic rocks with evolved rocks (e.g., ~3 wt % in TiO2 and ~3.0 in FeO*/MgO) suggest that the Poroshiri ophiolite was generated at fast-spreading ridge. Zr/Y and Zr/TiO2 ratios from the EPR basalts tend to decrease with increasing spreading rates. The Zr/Y and Zr/TiO2 ratios of the metabasaltic rocks are as low as those from the ODP Hole 1256D basalts generated at the EPR with an ultra-fast spreading rate. Assuming that the mantle source of the ophiolite was similar to that of the East Pacific Rise (EPR), this fact suggests that the spreading ridge produced the Poroshiri ophiolite spread at an ultra-fast rate more than 20 cm/year.<br>

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